As a method of controlling the number of heat source machines in a heat source system including a plurality of heat source machines, for example, a method disclosed in PTL 1 is known. PTL 1 discloses determining a load factor range where the coefficients of performance (COP) in individual inverter drive turbo heat source machines become equal to or greater than a predetermined value, respectively, and controlling the inverters so that the load factors of the individual inverter drive heat source machines fall within the determined load factor range.